Herein we demonstrate both the importance of Fe(I) in Negishi cross-coupling reactions with arylzinc reagents and the isolation of catalytically competent Fe(I) intermediates. These complexes, [FeX(dpbz)(2)] [X = 4-tolyl (7), Cl (8a), Br (8b); dpbz = 1,2-bis(diphenylphosphino)benzene], were characterized by crystallography and tested for activity in representative reactions. The complexes are low-spin with no significant spin density on the ligands. While complex 8b shows performance consistent with an on-cycle intermediate, it seems that 7 is an off-cycle species.

Ligands are essential for controlling the reactivity and selectivity of transition metal-catalyzed reactions. Access to large phosphine ligand libraries has become an essential tool for the application of metal-catalyzed reactions industrially, but these existing libraries are not well suited to new catalytic methods based on non-precious metals (i.e., Ni, Cu, Fe). The development of the requisite nitrogen- and oxygen-based ligand libraries lags far behind phosphines and the development of new libraries is anticipated to be time consuming. Here we show that this process can be dramatically accelerated by mining a typical pharmaceutical compound library that is rich in heterocycles for new ligands. Using this approach, we were able to screen a structurally diverse set of compounds with minimal synthetic effort and identify several new ligand classes for nickel-catalyzed cross-electrophile coupling. These new ligands gave improved yields for challenging cross-couplings of pharmaceutically relevant substrates compared to previously published ligands.

COGSWELL, P. M.; GOWER, N. J.; HADDOW, M. F.; HARVEY, J. N.; MURPHY, D. M.; NEEVE, E. C.; NUNN, J.; Angew. Chem., Int. Ed. 52 (2013) 4, 1285-1288, http://dx.doi.org/10.1002/anie.201207868 ; Sch. Chem., Univ. Bristol, Cantock's Close, Bristol BS8 1TS, UK; Eng.) -Roessler 23-030

Cross-electrophile coupling of aryl halides with alkyl halides has thus far been primarily conducted with stoichiometric metallic reductants in amide solvents. This report demonstrates that the use of tetrakis(dimethylamino)ethylene (TDAE) as an organic reductant enables the use of non-amide solvents, such as acetonitrile or propylene oxide, for the coupling of benzyl chlorides and alkyl iodides with aryl halides. Furthermore, these conditions work for several electron-poor heterocycles that are easily reduced by manganese. Finally, we demonstrate that TDAE addition can be used as a control element to 'hold' a reaction without diminishing yield or catalyst activity.

Iron phosphine complexes prove to be good precatalysts for the cross-coupling of alkyl, benzyl, and allyl halides with not only aryl triorganoborate salts but also related aluminum-, gallium-, indium-, and thallium-based nucleophiles. Mechanistic studies revealed that while Fe(I) can be accessed on catalytically relevant time scales, lower average oxidation states are not formed fast enough to be relevant to catalysis. EPR spectroscopic studies reveal the presence of bis(diphosphine)iron(I) complexes in representative catalytic reactions and related processes with a range of group 13 nucleophiles. Isolated examples were studied by Mossbauer spectroscopy and single-crystal X-ray structural analysis, while the electronic structure was probed by dispersion-corrected B3LYP DFT calculations. An EPR study on an iron system with a bulky diphosphine ligand revealed the presence of an S = 1 / 2 species consistent with the formation of a mono(diphosphine)iron(I) species with inequivalent phosphine donor environments. DFT analysis of model complexes allowed us to rule out a T-shaped Fe(I) structure, as this is predicted to be high spin. ■ RESULTS AND DISCUSSION Iron Phosphine Complexes for the Coupling of BR 4 − Nucleophiles. We previously found that the preformed Special Issue: Catalytic and Organometallic Chemistry of Earth-Abundant Metals